Osteoporosis is a condition in which bone tissue is reduced in amounts that increase the likelihood of fracture. Tamoxifen, an estrogen antagonist that is widely used to treat breast cancer, may be effective in preventing bone loss in postmenopausal women. This proposal is designed to further investigate the effects of tamoxifen on bone and mineral metabolism in postmenopausal women and in women with postmenopausal osteoporosis.